This invention relates to braking systems for commercial vehicles and particularly, but not exclusively, provides improvements in braking systems for use when the vehicle is reversing.
Systems are already known in which, when the vehicle is reversing, the brakes are applied automatically when an obstruction is detected in the path of the reversing vehicle.
In one arrangement, an obstruction is remotely detected by means of a proximity sensor that transmits a radio frequency signal to a cab-mounted receiver which generates an audible and/or visual signal that the vehicle is close to the object detected, which may be a person, another parked vehicle or another static obstruction.
In another arrangement, the brakes of a vehicle can be applied when an obstruction is detected, the brakes then being released after a pre-determined time or once the obstruction is removed or isolated. When the vehicle is an articulated lorry or truck comprising a tractor unit and a trailer, power for the activation system is generally supplied from the reversing light wiring on the trailer, whereby the system is armed whenever reverse gear is selected and the reversing light is on.
As described in GB2340902A, the electrical system on the trailer can include a battery back-up arrangement which supplies power to the activation system in the event that the electrical connections to the reversing lights are not in operation or otherwise disabled due to there being no electrical connection between tractor and trailer.
According to the arrangements described in GB2340902A, the battery back-up is activated by application of air pressure to release the park brake and supplies power to the emergency system when power from the tractor unit is isolated. When the tractor power supply is re-established, the battery of the battery back-up is recharged. However, there is a possibility that the back-up becomes activated unintentionally due, for example, to failure in an individual component which could result in activation of the armed system when travelling forward at speed.
Another system is also known to provide a brake application system which, when an obstruction is detected, will automatically apply the brakes and, after a pre-determined period of time, release the brakes to allow the reversing manoeuvre to continue. It is a feature of certain types of commercial vehicles, in particular buses and coaches when fitted with automatic gearboxes, that the gearboxes do not have a “park” selection mode to lock the transmission. They usually have only a forward, neutral and reverse gear selector.
In the know type of reversing system which applies the brakes for a pre-determined time, it could be dangerous if the know braking system was to become activated and the driver left his seat in order to investigate the reason for activation without deselecting reverse gear and applying the parking brake. After the pre-determined time had elapsed, the system would automatically release the brakes and, if reverse gear had not been de-selected, would allow the vehicle to continue reversing. This could cause an accident to happen which had been prevented in the first place by the automatic activation of the brakes for a pre-determined time.